Bolt Grades

I have been on a quite a few online email lists over the last 7 years or so, basically since they first came out. From the original Jeep-L list to the XJ-list to the Rockcrawler.com board, a common question comes up time and time again. No, I’m not talking about “how big a tire can I fit” or “which tire is better.” I’ll save those questions for the opinion section of everyone else’s website. I’m referring to the age-old question of “which fastener grade should I use?” It seems that everyone has an opinion on which grade is better but not many people can or will tell you why. Well, I’d like to explain the technical difference between a SAE Grade 8 (Grade 8) and a SAE Grade 5 (Grade 5) fastener. Most people think a bolt is a bolt is a bolt. They see it as a machined chunk of metal that holds or attaches things. Fasteners (aka bolts or screws) are complex mechanically-engineered hardware. They are made using different materials, different thread types (i.e. coarse, fine, extra fine), various lengths, with grip or no grip (shank), different types (i.e. hex, 12 pt, carriage, etc.), different coatings (i.e. passivated, cadmium, dry film lube, etc.), various classes of fit (i.e. class 3), and multiple grades (i.e. grade 5, 8, etc.). Bolts come with left or right hand threads, metric or SAE threads, different number of threads per inch (i.e. 20 or 28 for the same size fastener) and various versions of those (i.e. UNF versus UNJF). In addition, there are way too many military specs in existence to list them all here. So with all these differences, it’s no wonder most people don’t understand the difference between fasteners very well. Of all these differences, I’ll focus on the different grades since that is what most shade tree mechanics ask about. First, you need to be able to identify bolts by the different grades when you go to the local hardware store. Grade 5 bolts have 3 marks or lines on the head that are in the shape of a “Y”. Grade 8 bolts have 6 marks on the head. Second, the different grades have a meaning to them. It tells you how strong the fastener is. There are different types of strengths listed for each grade. Proof strength (about 90% of yield), ultimate tensile strength (bolt fails in stretch), yield strength (bolt begins to get a permanent set and changes crosssectional area typically) and shear strength (bolt prevents parts from separating by using it’s shank or body as a stop). Depending on how you are using the fastener, you would look at the appropriate and corresponding strength type. For example, bolts that attach a D-ring bracket to the bumper face of a vehicle would be critical in tension . So you would want to know what the tensile strength a particular bolt is. Bolts that attach winch-mounting plates are typically seeing mostly shear loads thus preventing the winch from departing from the vehicle during winching operations. In that case, shear strength is important to you. Mark’s Standard Handbook for Mechanical Engineers lists Grade 5 fasteners as 120 ksi fasteners. This means the tensile strength is 120,000 lbs per square inch. It also lists Grade 8’s as 150 ksi fasteners meaning the tensile strength is 150,000 lbs per square inch. Also, the ultimate shear strength of a fastener is typically about 60% of its ultimate tension strength. So given a certain diameter (cross-sectional area) and strength rating, someone can figure out how much load that fastener can carry in both tension and shear.

Using a .000 lbs / in2 x .Example of bolts under single shear load
Example of bolts under tension
Let’s look at an example of where grade 5 and grade 8 bolts are subjected to single shear loads (winch plate reference). use area of a circle) = Pi x r2 where R (radius) = .000 lbs / in2 x . So you need to know the nominal minor diameter of that particular fastener. We can now calculate the A (area) of the cross-section: A = Pi x r2 = Pi x (. fasteners are not brittle below 180 ksi ultimate tensile strength. Going back to the D-ring on the face of the bumper example. Typically. Well. Grade 5’s have an ultimate tensile strength of 120 ksi and a grade 8 fastener has an ultimate tensile strength of 150 ksi.250028-UNF at . 17-4 PH and 17-7 PH.0491 in2 = 4468 lbs Using the same . Nearly all fasteners are considered ductile except some made from PH 15-6 Mo.125. It is titled “Screw threads. Calculating the tensile capability is not as easy as shear since the thinnest portion of the bolt is at the minor diameter of the threads (bottom of the thread “V”).0491 square inches (in2) Capability in shear = 91.250-inch diameter grade 5 fastener results in the following: Capability in shear = 75. first you need to understand what the term “brittle” really means.250-inch diameter grade 8 fastener gives you the following shear capability: A = Cross-sectional area of the fastener size (since bolt bodies/shanks have circular cross-sections. MIL-S-8879C lists the nominal minor diameter of a .0491 in2 = 3683 lbs That’s a difference of over 750 lbs or over 1/3 ton. you would want to know its tensile carrying capability.250/2 = . therefore A = Pi x (.2065/2)2 = . I’ve also heard the argument that grade 8’s are more brittle than grade 5’s and that’s why you shouldn’t use them. In this example you can clearly see that using a grade 8 fastener has a superior advantage over the grade 5. This is why brittle is a relative term.2065 inches. It lists that and a lot more for almost all possible fasteners.03349 in2
. controlled radius root with increased minor diameter. they will fail almost 800 lbs earlier. general specification for”. Brittleness in bolts is defined as failure at stresses apparently below the strength of the bolt material with little or no evidence of plastic deformation. Therefore the result is if someone is using grade 5 bolts in a shear application like the winch plate example.125)2 = . That’s where military specification MIL-S-8879C comes in.

03349 in2 = 4019 lbs minimum Again. clean threads.000 lbs / in2 x .2500 28 Tension Capability (lb) Shear Capability (lb) 0. Based on this. 0. the argument that you should not use grade 8’s because they are more brittle than grade 5’s is not a true statement in most applications. Grade 5’s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. The grade 5 fastener has already reached its ultimate load and FAILED BEFORE the grade 8 starts to yield or stretch.1640 thrds/in 32 Tension Capability (lb) Shear Capability (lb) 0.
SAE Grade 5 Ultimate Tensile Capability of Fastener (ksi) Ultimate Shear Capability of Fastener (ksi) Fastener Diameter in. Due to space and time limitations.3125 24 Tension Capability (lb) Shear Capability (lb) 0. According to Marks’ Standard Handbook for Mechanical Engineers.03349 in2 = 5024 lbs minimum Grade 5 bolt capability in yield (stretch) = 92. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure.000 lbs / in2 x . Here is a quick point about fastener fatigue. you can see that the grade 8 will support over 1000 lbs more or a 1/2-ton more.000 lbs / in2 x . here is a chart showing you the tension and shear minimum capabilities of different grade fasteners relative to their size. Therefore. you want to make sure you torque your fasteners to the appropriate level using a torque wrench and make sure to torque dry.03349 in2 = 4354 lbs minimum Grade 8 bolt capability in tension (failure) = 150. Far more than you will ever winch or pull on.03349 in2 = 3081 lbs minimum Grade 5 bolt capability in tension (failure) = 120.000 lbs / in2 x . But there’s something more important to note.3750 24 Tension Capability (lb) Typical Material 120 75 Med Carbon Steel 1468 1584 2169 2126 4007 3682 6440 5750 9888 SAE Grade 8 150 91 Med Carbon Alloy Steel 1835 1922 2711 2580 5009 4470 8050 6980 12360 ARP Fastener 160 95 A286 CRES 1957 2007 2892 2694 5340 4660 8590 7290 13180 SPS Fastener 180 108 A286 CRES 2202 2281 3253 3062 6010 5300 9660 8280 14830 MS14181 220 132 SPS Fastener 260 156 MP35N Super Alloy 3181 3295 4699 4420 8682 7660 13953 11970 21423
Inconel 718 2691 2788 3976 3743 7347 6480 11807 10120 18127
. grade 8’s are tougher than grade 5’s. Fatigue usually doesn’t play a big part in grade 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Therefore. Toughness is an important feature of a fastener.1900 32 Tension Capability (lb) Shear Capability (lb) 0. Rockwell …) the harder the material is and the tougher it is to damage.Grade 8 bolt capability in yield (stretch) = 130. It is the opposite of brittleness and gives you an idea of how it will handle abuse without being damaged and eventually weakening the fastener or can cause fatigue to appear much earlier than normal. Lubricated threads significantly change the actual preload on the fastener and you risk over torquing it. One way to “measure” toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell.

Industry practice is to apply a safety factor to address any unknowns and/or combined load cases to give you an adequate margin of safety.0000 12 Tension Capability (lb) Shear Capability (lb)
8280 13338 11270 18139 14730 23028 18640 29218 23010 42726 33130 58434 45100 75968 58900
10050 16673 13680 22674 17870 28785 22610 36524 27920 53408 40200 73043 54700 94961 71500
10490 17780 14280 24190 18650 30700 23610 38960 29150 57000 42000 77900 57100 101300 74600
11930 20010 16240 27210 21210 34540 26840 43800 33130 64100 47700 87700 64900 114000 84800
14580 24453 19840 33255 25920 42218 32800 53568 40500 78331 58300 107129 79400 139275 103700
17230 28899 23450 39302 30770 49894 38770 63307 47900 92573 68900 126607 93800 164598 122500
These examples show how much of a load can be carried by the fastener BUT you need to make sure the parent material is strong enough to handle the loads.6250 18 Tension Capability (lb) Shear Capability (lb) 0. as an individual who has spent some serious coin on a winch or lift kit. the difference in a few cents per fastener adds up to a lot for them.5000 20 Tension Capability (lb) Shear Capability (lb) 0.
. “which fastener grade should I use?” I hope it’s very clear by now that grade 8 fasteners are far superior to grade 5 fasteners.7500 16 Tension Capability (lb) Shear Capability (lb) 0. Getting back to the original question. Corrosion can cause a fastener to fail well below its initial strength. as well.4375 20 Tension Capability (lb) Shear Capability (lb) 0. So be smart and use only new fasteners when installing or reinstalling some cool new widget on your rig.8 and 10.8750 14 Tension Capability (lb) Shear Capability (lb) 1. otherwise it will fail.5625 18 Tension Capability (lb) Shear Capability (lb) 0. Since the OEM’s manufacture millions of vehicles each year. Another good point to make is to never reuse fasteners after they have been subjected to loading or the elements.9). If this is so. then why do the automotive manufacturers use some grade 5 fasteners? The automotive OEM’s use what it needs to be safe and nothing more since there is a difference in cost between grade 5 and grade 8 (or metric 8. However. I wouldn’t let the few cents difference in the cost of a grade 8 versus a grade 5 fastener make up my mind as to which fastener I would use.Shear Capability (lb) 0.